The present invention relates to a gamma ray detector suitable for use in a positron CT (namely, a positron computed tomography scanner) for measuring the distribution of a positron emitting nuclide injected into a human body.
When a positron comes to rest in matter, it is quickly annihilated by an electron into a pair of photons, namely, gamma rays. Gamma ray detectors used in a positron CT detect such gamma rays, and are usually made up of single crystal scintillators of bismuth germanate Bi.sub.4 Ge.sub.3 O.sub.12 (hereinafter referred to as "BGO") and a photomultipliers. The bismuth germanate BGO has an effective atomic number of 74, a density of 7.1 g/cm.sup.3, and a large absorption coefficient for gamma rays (of 511 KeV). Accordingly, a positron CT provided with the BGO scintillation is excellent in geometrical resolution. However, the luminescence decay constant of BGO is long, that is, is 300 ns (nano seconds), and therefore the time resolution of the positron CT is not good, that is, has a value of 3 to 4 ns. (Refer to an article by Katsumi Takami, appearing on page 122 of the Feb. 18, 1980 issue of Nikkei Electronics).
In order to improve the time resolution of the positron CT, the gamma ray detector is required to use a scintillator having a short luminescence decay constant.